Long-term variations in major air pollutants
To understand the trends and characteristics of air pollution in Ahvaz in recent years, an analysis of air quality and air pollutant concentrations from 2013 to 2018 was carried out. We analyzed the long-term trends of air pollutants including SO2, NO2, O3, CO, PM10, and PM2.5 from three monitoring stations. The pollutant shows the annual mean, averaged over all included sites that had annual data capture greater than 50%.
Among the stations analyzed, the highest average concentrations for PM were found in Naderi and Edareh Kol stations, respectively. Naderi station presented the highest averages for gaseous pollution (i.e., SO2, NO2, O3, and CO), probably due to the station's proximity to mobile sources. Padad station presented the lowest average concentrations for all pollutants. The results are summarized in Table 2 and Fig. 2.
Table 2
Air quality stations in Ahvaz, measured pollutants concentrations and basic statistic parameters from the 2013 to 2021 dataset.
Station
|
Pollutants
|
Mean (S. D.)
|
Max.- Min.
|
Edareh Kol
|
So2
|
11.92 (5.69)
|
0.00- 48.65
|
NO2
|
24.89 (17.26)
|
1.00- 137.17
|
O3
|
16.42 (10.64)
|
0.00- 54.05
|
CO
|
1.11 (0.88)
|
0.11–11.73
|
PM10
|
136.10 (137.51)
|
5.26–2309
|
PM2.5
|
46.66 (55.21)
|
0.56- 1516.05
|
Naderi
|
So2
|
19.79 12.74)
|
0.01- 182.79
|
NO2
|
32.88 (30.18)
|
1.00- 252.38
|
O3
|
30.44 (16.75)
|
1.00- 194.27
|
CO
|
1.82 (1.23)
|
0.06–11.61
|
PM10
|
173.69 (200.82)
|
19.12- 3477.41
|
PM2.5
|
59.36 (107.28)
|
7- 3399.42
|
Padad
|
So2
|
13.15 (7.54)
|
6.32–44.15
|
NO2
|
22.74 (9.15)
|
7.10- 46.32
|
O3
|
9.22 (0.34)
|
8.75–10.07
|
PM10
|
109.88 (78.43)
|
35.89- 454.61
|
PM2.5
|
33.42 (19.47)
|
13.80- 136.68
|
The highest annual NO2 (68.63 ppb), O3 (40.65 ppb), and PM2.5 (70.56 µg/m − 3) mean concentrations were observed in 2015, whereas the highest annual mean concentration of SO2 (22.14 ppb), PM10 (219.82 µg m3), and CO (7.32 ppm) recorded in 2013. Annual PM10 and PM2.5, mean concentrations were higher than the World Health Organization air quality guideline levels during the entire study period (see Fig. 3)
As depicted in Table 3, the study found a statistically significant correlation between SO2, NO2, and CO. However, the relationship was inverse for NO2 and CO (-0.31, P-value = 0.00). Expectedly strong association between NO2 and O3 (r = 0.42, P-value = 0.00).
Table 3
Correlation coefficient with 95% confidence interval between SO2, NO2, O3, CO, PM10, and PM2.5 in Ahvaz Iran
|
NO2
|
SO2
|
PM10
|
PM2.5
|
O3
|
CO
|
NO2
|
1
|
|
|
|
|
|
SO2
|
0.131
(0.095, 0.166)
|
1
|
|
|
|
|
PM10
|
0.045
(0.009, 0.082)
|
0.054
(0.018, 0.090)
|
1
|
|
|
|
PM2.5
|
0.098
(0.062, 0.134)
|
0.086
(0.050, 0.122)
|
0.374
(0.342, 0.405)
|
1
|
|
|
O3
|
0.424
(0.394, 0.453)
|
0.031
(-0.005, 0.067)
|
-0.014
(-0.050, 0.023)
|
0.019
(-0.017, 0.055)
|
1
|
|
CO
|
-0.310
(-0.343, -0.277)
|
0.295
(0.261, 0.328)
|
0.018
(-0.019, 0.054)
|
0.073
(0.036, 0.108)
|
-0.450
(-0.478, -0.421)
|
1
|
SO2
Data presented in Fig. 2A have not observed a significantly changed trend for SO2 annual concentrations from 2013 to 2021, showing a very small downward trend. In 2018, SO2 levels dipped below 12 ppb, which were approximately 50 percent less than values reported in the 2013s (22.14 ± 5.97). The trends in SO2 levels showed a gradual rise after 2018.
SO2 levels remain constant all through the week for the entire study period (ranging from 15.16 to 15.88). Even we did not find a significant seasonal variation in SO2 levels (Table 4).
Table 4
Descriptive statistics for air pollution, across seasons, in Ahvaz, Iran, 2013–2021
Pollutants
|
Spring
|
Summer
|
Autumn
|
Winter
|
Total
|
SO2
|
15.06 ± 7.94
|
17.00 ± 8.64*
|
15.24 ± 6.92
|
14.83 ± 8.02
|
15.54 ± 7.95
|
NO2
|
24.00 ± 19.35
|
24.80 ± 20.24
|
25.90 ± 18.23
|
27.93 ± 25.40*
|
25.63 ± 20.99
|
O3
|
21.22 ± 13.11*
|
20.41 ± 13.24
|
18.70 ± 19.00
|
20.86 ± 11.19
|
20.30 ± 14.44
|
CO
|
2.53 ± 2.98*
|
2.24 ± 2.28
|
2.18 ± 1.53
|
2.10 ± 1.97
|
2.27 ± 2.26
|
PM10
|
151.14 ± 93.41
|
165.78 ± 114.60
|
164.60 ± 172.11
|
176.00 ± 202.58*
|
164.26 ± 151.48
|
PM2.5
|
48.30 ± 24.97
|
57.57 ± 42.47*
|
55.46 ± 70.92
|
52.75 ± 49.13
|
53.51 ± 49.57
|
Values are mean ± SD
* Friedman test P < 0.001
|
NO2
Based on the data (see Fig. 2), the NO2 levels indicate an insignificant fluctuation between 2013 and 2021. Given the NO2 concentration values from 2013, the maximum annual values of 68.63 ppb (25th and 75th percentile: 52.13, 81.35) were recorded in 2015. Although, the average annual change (AAC) of NO2 was 21.79 ppb, in the" Time series model with trend line fitting", NO2 shows a very minor downtrend with a coefficient of -0.007 and then an upward trend in the forecast part. We did not observe seasonal variation in NO2 concentration for overall 8 years (see Table 1)
302 days (10%) of the study period NO2 concentration were above 53 ppb. A worse situation was recorded for the year 2015 with 269 days (73% of days in the year) with extreme values.
O3
Ozone, with annual mean concentrations of 20.30 ppb (25th and 75th percentiles, 10.78–24.86), In the "time-series diagram with trend line fitting", the O3 pollutant shows a slight downward trend in the graph with a coefficient of -0.004 and then an upward trend in forecast part.
The daily average O3 concentrations showed an upward trend since 2015. Especially, in 2015 Ahvaz experienced a remarkable increase leading to the highest values (40.65 ppb; 28.58, 47.15) and then slight fluctuation until 2021. Overall, simply an eight-year observation of O3 recorded an AAC of + 37.67 ppb. No significant differences were observed in monthly mean values of O3. In the overlay, we did not find significant seasonality in O3.
PM10
A fluctuating trend in PM10 levels has been found between 2013 and 2021 (see Fig. 2). However, PM10 concentration significantly dropped in 2018 (= 113.81 ± 48.64), and compared to 2013, the levels of PM10 in 2020 were 55.2% lower (PM10 = 121.71 ± 101.22). Overall, AAC of PM10 was − 4.06 µg/m3 observed with a negative trend even in the forecast part.
However, the annual PM10 level was higher than the WHO and national standard levels (10 µg/m3 for WHO and 50 for National). Also, we found that in 38.7% of the study period (1130 days) the PM10 levels exceeded the threshold daily value of 150 µg/m3. Between 2013 and 2021, percent PM10 concentration that exceeded WHO guideline of 24 hours were 59.8%, 38.9%, 54.8%, 41.0%, 45.8%, 52.9%, 13.7%. and 18.9% respectively.
Throughout the study period, statistically significant higher mean PM10 levels were observed on Saturdays than on weekdays (P < 0.05)
PM2.5
Particulate Matter with diameter ≤ 2.5 µm3 levels showed very comparable trends to PM10 pollution with a peak annual concentration of 70.56 µg/m3 in 2015. The mean concentration of PM2.5 was slightly higher in summer (57.57 ± 42.47 µg /m3) in comparison with Spring, Autumn, and Winter seasons (see Table 1). We also calculated the ratio of PM2.5/PM10 which represents the composition of particulate pollution.
Specifically, there was a clear downward trend in PM2.5 levels from 2013 with decreases ranging from 10 to 16 µg/m3 from the end of 2018 onward. Overall, AAC of -5.65 was calculated for 8 years PM2.5.
We observed continuously decreasing trends in the PM2.5/PM10 ratio from 2013 in Ahvaz (from 44% in 2013 to 36% in 2020), because of serious PM10 pollution (shown in Fig. 2). Although the pollution level was overall reduced, the city did not meet the national daily standard of 35 µg/m3 for two-thirds of the study period and almost entirely of the study (98.70%) as per the annual threshold level of WHO guidelines.